ACI 318 08 Seismic Requirements L E Garcia

ACI 318-08 - Seismic Requirements -- Luis E. Garcia

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Seismic DesignSeismic DesignSeismic Design Requirements in ACI 318-08

Seismic Design Requirements in ACI 318-08

By: Luis Enrique García

President American Concrete Institute – ACI – 2008-2009Partner Proyectos y Diseños Ltda. Consulting Engineers

Professor Universidad de los AndesBogotá, Colombia

By: Luis Enrique García

President American Concrete Institute – ACI – 2008-2009Partner Proyectos y Diseños Ltda. Consulting Engineers

Professor Universidad de los AndesBogotá, Colombia

Chapter 1Chapter 1Chapter 1General Requirements

Chapter 1General Requirements

Modifications inModifications inScopeTerminologyScopeTerminology

R1.1.9 – Provisions for earthquake resistanceR1.1.9 – Provisions for earthquake resistance

Commentary was expanded to:

Explain changes in terminology usedSimplify adoption and interaction of

Commentary was expanded to:

Explain changes in terminology usedSimplify adoption and interaction ofSimplify adoption and interaction of ACI 318-08 with model codes and other documents

Simplify adoption and interaction of ACI 318-08 with model codes and other documents

R1.1.9 – Provisions for earthquake resistanceR1.1.9 – Provisions for earthquake resistance

In this version of ACI 318 (2008), for the firstIn this version of ACI 318 (2008), for the firstIn this version of ACI 318 (2008), for the firsttime, earthquake resistance requirements aredefined in function of the Seismic DesignCategory — SDC required for the structure andnot directly associated with the seismic riskzone.

In this version of ACI 318 (2008), for the firsttime, earthquake resistance requirements aredefined in function of the Seismic DesignCategory — SDC required for the structure andnot directly associated with the seismic riskzone.

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The minimum SDC to use is governed by thelegally adopted general building code of whichACI 318 forms a part.

The minimum SDC to use is governed by thelegally adopted general building code of whichACI 318 forms a part.


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TABLE R1.1.9.1 — CORRELATION BETWEEN SEISMIC-RELATED TERMINOLOGY IN MODEL CODES

Code, standard, or resourcedocument and edition

Level of seismic risk or assigned seismicperformance or design categories as

defined in the Codedefined in the Code

ACI 318-08; IBC 2000, 2003; 2006; NFPA 5000, 2003, 2006; ASCE 7-

98, 7-02, 7-05; NEHRP 1997, 2000, 2003

SCD*A, B

SCSC

SCDD, E, F

BOCA National Building Code 1993, 1996, 1999; Standard

Building Code 1994, 1997, 1999; ASCE 7-93, 7-95; NEHRP 1991,

SPC†

A, BSPC

CSPC D; E

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ASCE 7 93, 7 95; NEHRP 1991, 1994

Uniform Building Code 1991, 1994, 1997

Seismic Zone0, 1

Seismic Zone2

Seismic Zone3, 4

*SDC = Seismic Design Category as defined in code, standard, or resource document.†SPC = Seismic Performance Category as defined in code, standard, or resource document

Chapter 2 Notation and Definitions

Chapter 2 Notation and Definitions

There were important changes in notation of the whole document and all individual Chapter notation was moved to Chapter 2.

There are a few new definitions related

There were important changes in notation of the whole document and all individual Chapter notation was moved to Chapter 2.

There are a few new definitions relatedThere are a few new definitions related to Chapter 21. All definitions, old and new, were moved to Chapter 2.There are a few new definitions related to Chapter 21. All definitions, old and new, were moved to Chapter 2.

Chapter 21Earthquake-resistant

structures

Chapter 21Earthquake-resistant

structuresstructuresstructuresChapter 21 was reorganized in function of Seismic Design Categories (SDC) A, B, C, and D, E, and F in incremental order from ordinary to special:

Chapter 21 was reorganized in function of Seismic Design Categories (SDC) A, B, C, and D, E, and F in incremental order from ordinary to special:

A → B → C → D, E, FA → B → C → D, E, F

Seismic Design Category and Energy Dissipation Capacity

Seismic Design Category and Energy Dissipation Capacity

SDCS i i D i

Denomination(E di i ti Must comply with inSeismic Design

Category(Energy dissipation

capacity)Must comply with in

ACI 318-08

AOrdinary

Chapters 1 to 19 and 22

B Chapters 1 to 19, 22, and 21.2

C Intermediate Chapters 1 to 19, 22, and 21.3 y 21.4

D, E, F Special Chapters 1 to 19, 22,And 21.5 to 21.13


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ACI 318-08 – Chapter 21 Earthquake-resistant structures

ACI 318-08 – Chapter 21 Earthquake-resistant structures

Content21.1 – General requirements21 2 Ordinary moment frames

Content21.1 – General requirements21 2 Ordinary moment frames B21.2 – Ordinary moment frames21.3 – Intermediate moment frames21.4 – Intermediate precast structural walls21.5 – Flexural members of special moment frames21.6 – Special moment frame members subjected to bending and

axial load21.7 – Joints of special moment frames21.8 – Special moment frames constructed using precast concrete21.9 – Special structural walls and coupling beams

21.2 – Ordinary moment frames21.3 – Intermediate moment frames21.4 – Intermediate precast structural walls21.5 – Flexural members of special moment frames21.6 – Special moment frame members subjected to bending and

axial load21.7 – Joints of special moment frames21.8 – Special moment frames constructed using precast concrete21.9 – Special structural walls and coupling beams

BC

D

E21.10 – Special structural walls constructed using precast concrete

21.11 – Structural diaphragms and trusses21.12 – Foundations21.13 – Members not designated as part of the seismic-force-

resisting system

21.10 – Special structural walls constructed using precast concrete

21.11 – Structural diaphragms and trusses21.12 – Foundations21.13 – Members not designated as part of the seismic-force-

resisting system

E

F

21.1 – General requirements21.1 – General requirementsScope Scope

Chapter 21 contains provisions considered to be the minimum requirements for a cast-in-place or precast concrete structure capable of sustaining a series of oscillations into the inelastic range of response without critical deterioration in strength.

Chapter 21 contains provisions considered to be the minimum requirements for a cast-in-place or precast concrete structure capable of sustaining a series of oscillations into the inelastic range of response without critical deterioration in strength.g

Therefore, the objective is to provide energy dissipation capacity in the nonlinear range of response.

g

Therefore, the objective is to provide energy dissipation capacity in the nonlinear range of response.

TABLE R21.1.1 — SECTIONS OF CHAPTER 21 TO BE SATISFIED IN TYPICAL APPLICATIONS

Component resistingearthquake effect,

unlessotherwise noted

Seismic Design Category (SDC)

A(none)

B(21.1.1.4)

C(21.1.1.5)

D(21.1.1.6)

Analysis and design requirements 21.1.2 21.1.2 21.1.2, 21.1.3requirements

None

Materials None None 21.1.4 21.1.7

Frame members 21.2 21.3 21.5, 21.6, 21.7, 21.8

Structural walls and coupling beams None None 21.9

Precast structural walls None 21.4 21.4,† 21.10

Structural diaphragms and trusses None None 21.11trusses

Foundations None None 21.12

Frame members not proportionedto resist forces induced by earthquake motions

None None 21.13

Anclajes None 21.1.8 21.1.8

Global Energy Dissipation CapacityGlobal Energy Dissipation Capacity

ForceForce elasticelastic

FeFemaximum elastic displacement demandmaximum elastic displacement demand

Maximum elastic force demand

Maximum elastic force demand

DisplacementDisplacement

FyFy

uu uuuu

nonlinearnonlinear

Maximum nonlinear displacement demandMaximum nonlinear displacement demandYield strengthYield strength

DisplacementDisplacementuyuy umumueue

In several earthquake resistance regulations this is defined through parameter R

In several earthquake resistance regulations this is defined through parameter R

e e

y y

F uRF u

= =


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Elastic vs. Nonlinear DemandElastic vs. Nonlinear Demand

1010

2020

uu

linear elasticlinear elasticnonlinearnonlinear

-20-20

-10-10

0000 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515

time (s)time (s)

uu(cm)(cm)

linear elasticlinear elastic

nonlinearnonlinear0 20 20.40.40.60.60.80.8

forceforce

-0.8-0.8-0.6-0.6-0.4-0.4-0.2-0.2

000.20.2

00 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515

time (s)time (s)

forceforce

(1/W)(1/W)

Current seismic design strategyCurrent seismic design strategyGiven an energy dissipation capacity for the structural material and structural system, defined through an Rvalue depending of the detailing scheme the design

Given an energy dissipation capacity for the structural material and structural system, defined through an Rvalue depending of the detailing scheme the designvalue depending of the detailing scheme the design horizontal seismic force is obtained from:

and the maximum elastic force demand is in turn

value depending of the detailing scheme the design horizontal seismic force is obtained from:

and the maximum elastic force demand is in turn

ey

FFR

=

and the maximum elastic force demand is in turn obtained using Newton’s 2nd Law:and the maximum elastic force demand is in turn obtained using Newton’s 2nd Law:

= ×e aF m ass S T( , )ξ Acceleration response spectrum from the general building codeAcceleration response spectrum from the general building code

What would happen ifWhat would happen ifWhat would happen if energy dissipation

capacity is not available?

What would happen if energy dissipation

capacity is not available?available?available?


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Nonstructural wallpanel in contact with the structure column

ACI 318-08 requires (21.1.2) that interaction between structural and nonstructural elements that may affect the response during the earthquake

hNonstructural wallpanel separated from the structure

o st uctu a e e e ts t at ay a ect t e espo se du g t e ea t qua eMust be taken into account. Rigid members assumed not to be a part of the seismic-force-resisting system are permitted provided their effect on the response of the system is considered and accommodated in the structural design. Consequences of failure of structural and nonstructural members that are not a part of the seismic-force-resisting system shall be considered.

C.21.1 – General RequirementsC.21.1 – General Requirements

Compressive strength of concrete 21 MPaSpecified compressi e strength of light eightCompressive strength of concrete 21 MPaSpecified compressi e strength of light eight

cf ′ ≥Specified compressive strength of lightweight concrete ≤ 35 MPaFor computing the amount of confinement reinforcement fyt ≤ 700 MPa (= 100,000 psi = 7000 kgf/cm2)Reinforcing steel must meet ASTM A706. If ASTM A615 is used, it must meet:

Specified compressive strength of lightweight concrete ≤ 35 MPaFor computing the amount of confinement reinforcement fyt ≤ 700 MPa (= 100,000 psi = 7000 kgf/cm2)Reinforcing steel must meet ASTM A706. If ASTM A615 is used, it must meet:

The actual yield strength based on mill tests does not exceed fy by more than 125 MPa.The ratio of the actual tensile strength to the actual yield strength is not less than1.25

The actual yield strength based on mill tests does not exceed fy by more than 125 MPa.The ratio of the actual tensile strength to the actual yield strength is not less than1.25


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Reinforcing steelReinforcing steelstressMPa

stressMPa

σσactual tensile strengthactual tensile strength

σσ

failurefailureσσyy

σσuu

actual yield strengthactual yield strength

strainstrain

EE11

OOmaxmax

maximum elongationmaximum elongation

yy

yield elongationyield elongation

εεεεεε

21.2 – Ordinary moment frames21.2 – Ordinary moment framesCorresponds to SDC BBeams must have at least two continuousCorresponds to SDC BBeams must have at least two continuousBeams must have at least two continuous longitudinal bars along both top and bottom faces. These bars shall be developed at the face of support.Columns having clear height less than or equal to

Beams must have at least two continuous longitudinal bars along both top and bottom faces. These bars shall be developed at the face of support.Columns having clear height less than or equal to

c1c2

five times the dimension c1must be designed for shear in accordance with 21.3.3. (shear requirements for intermediate SDC C)

five times the dimension c1must be designed for shear in accordance with 21.3.3. (shear requirements for intermediate SDC C)

21.3 - Intermediate moment frames21.3 - Intermediate moment framesRequirements for this Section are equivalent to the rest of Chapter 21, but are less strict and have a lesser scope.

Requirements for this Section are equivalent to the rest of Chapter 21, but are less strict and have a lesser scope.p

Two alternatives are presented for shear design of beams and columns:

Obtain design shear forces as function of nominal end moments as done for special elements, oruse twice the shear from analysis This is

p

Two alternatives are presented for shear design of beams and columns:

Obtain design shear forces as function of nominal end moments as done for special elements, oruse twice the shear from analysis This isuse twice the shear from analysis. This is equivalent to using the following load combinations:

U = 1.2D + 1.0L + (1.0E)x2.0U = 0.9D + (1.0E)x2.0

use twice the shear from analysis. This is equivalent to using the following load combinations:

U = 1.2D + 1.0L + (1.0E)x2.0U = 0.9D + (1.0E)x2.0

21.3 - Intermediate moment frames21.3 - Intermediate moment framesReinforcement details in a frame member shall satisfy beam requirements if the f d i l i l d P dReinforcement details in a frame member shall satisfy beam requirements if the f d i l i l d P d

y qfactored axial compressive load, Pu , does not exceed .

When Pu is greater reinforcing details must meet column requirements.

y qfactored axial compressive load, Pu , does not exceed .

When Pu is greater reinforcing details must meet column requirements.

g cA f 10′

When a slab-column system without beams is part of the seismic-force-resisting system, reinforcement details in any span resisting moments caused by E must satisfy 21.3.6.

When a slab-column system without beams is part of the seismic-force-resisting system, reinforcement details in any span resisting moments caused by E must satisfy 21.3.6.


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21.3 - Intermediate moment frames21.3 - Intermediate moment framesFor beams:

Moment strength must comply with:For beams:

Moment strength must comply with:g p yg p y

( )≥ ⋅n n faceM M max .15

nM −nM −

n nM M13

+ −≥

21.3 - Intermediate moment frames21.3 - Intermediate moment framesFor beams:

At both ends of the beam, hoops shall be provided over

For beams:

At both ends of the beam, hoops shall be provided over lengths not less than 2h measured from the face of the supporting member toward midspan. The first hoop shall be located not more than 50 mm from the face of the supporting member. Spacing of hoops shall not exceed the smallest of d/4, 8db of the smaller longitudinal bar, 24dbof hoop, or 300 mm. Stirrups shall be spaced not more than d/2 throughout the length of the beam.

lengths not less than 2h measured from the face of the supporting member toward midspan. The first hoop shall be located not more than 50 mm from the face of the supporting member. Spacing of hoops shall not exceed the smallest of d/4, 8db of the smaller longitudinal bar, 24dbof hoop, or 300 mm. Stirrups shall be spaced not more than d/2 throughout the length of the beam.

2h 2h@d/2

21.3 - Intermediate moment frames21.3 - Intermediate moment framesFor columns

At both ends of the column, hoops shall be provided at spacing so over a length measured from the joint

For columnsAt both ends of the column, hoops shall be provided at spacing so over a length measured from the jointa length 0 measured from the joint face. Spacing so shall not exceed the smallest of 1/2 of the smallest cross-sectional dimension of the column, 8db of the smallest longitudinal bar enclosed, 24db of the hoop bar, or 300 mm. Outside this length spacing must be the one defined in Chapters 7 and 11

a length 0 measured from the joint face. Spacing so shall not exceed the smallest of 1/2 of the smallest cross-sectional dimension of the column, 8db of the smallest longitudinal bar enclosed, 24db of the hoop bar, or 300 mm. Outside this length spacing must be the one defined in Chapters 7 and 11defined in Chapters 7 and 11.Length 0 shall not be less than the largest of the maximum cross-sectional dimension of the column;, 1/6 of the clear span of the column, or 450 mm.

defined in Chapters 7 and 11.Length 0 shall not be less than the largest of the maximum cross-sectional dimension of the column;, 1/6 of the clear span of the column, or 450 mm.

21.3 - Intermediate moment frames21.3 - Intermediate moment framesTwo-way slabs without beams (slab-column frames)

Reinforcement provided to resist Mslab shall be placed ithi th l t i

Two-way slabs without beams (slab-column frames)

Reinforcement provided to resist Mslab shall be placed ithi th l t i slabwithin the column strip.

Not less than 50% of the reinforcement in the column strip at supports shall be placed within the effective slab width defined by lines drawn parallel to the span at 1.5 slab depths from the column face .

Continuous bottom reinforcement in the column strip shall be not less than 33% of the top reinforcement at th t i th l t i

slabwithin the column strip.

Not less than 50% of the reinforcement in the column strip at supports shall be placed within the effective slab width defined by lines drawn parallel to the span at 1.5 slab depths from the column face .

Continuous bottom reinforcement in the column strip shall be not less than 33% of the top reinforcement at th t i th l t ithe support in the column strip.

Not less than 25% of the top reinforcement at the support in the column strip shall be continuous throughout the span.

the support in the column strip.

Not less than 25% of the top reinforcement at the support in the column strip shall be continuous throughout the span.


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21.3 - Intermediate moment frames21.3 - Intermediate moment frames 21.3 - Intermediate moment frames21.3 - Intermediate moment framesTwo-way slabs without beams (slab-column frames)Two-way slabs without beams (slab-column frames)

At the critical sections for punching shear, shear caused by factored gravity loads shall not exceed,

where must be calculated as defined in Chapter 11 for prestressed and non prestressed slabs.

At the critical sections for punching shear, shear caused by factored gravity loads shall not exceed,

where must be calculated as defined in Chapter 11 for prestressed and non prestressed slabs.

cV0.4φ cV

This requirement may be waived if the slab complies with 21.13.6This requirement may be waived if the slab complies with 21.13.6

21.4 — Intermediate precast structural walls21.4 — Intermediate precast structural walls

Requirements of 21.4 apply to intermediate t t t l ll f i t f th

Requirements of 21.4 apply to intermediate t t t l ll f i t f thprecast structural walls forming part of the

seismic-force resisting systems.

In connections between wall panels, or between wall panels and the foundation, yielding must be restricted to steel elements or reinforcement..

precast structural walls forming part of the seismic-force resisting systems.

In connections between wall panels, or between wall panels and the foundation, yielding must be restricted to steel elements or reinforcement..

Elements of the connection that are not designed to yield must develop at least 1.5Sy.Elements of the connection that are not designed to yield must develop at least 1.5Sy.

21.5 — Flexural members of specialmoment frames


A i l f P t t dA i l f P t t d f A0 10 ′Axial force Pu must not exceedClear span of element n must be larger than 4dRatio bw/h > 0.3Width b must comply with:

Axial force Pu must not exceedClear span of element n must be larger than 4dRatio bw/h > 0.3Width b must comply with:

c gf A0.10 ′

Width bw must comply with:bw > 250 mmlarger than the width of the supporting element plus 3h/4 at each side

Width bw must comply with:bw > 250 mmlarger than the width of the supporting element plus 3h/4 at each side


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Longitudinal reinforcementLongitudinal reinforcementLongitudinal reinforcement

Steel ratio for negative and positive reinforcement must not be less than:

Longitudinal reinforcement

Steel ratio for negative and positive reinforcement must not be less than:

c

y y

ff f

1.44

ρ′

≥ ≥⋅

but:

At least two bars continuous top and bottom.

but:

At least two bars continuous top and bottom.

y yf f4

0.025ρ ≤



Longitudinal reinforcementMoment strength at each section must be at least:

Longitudinal reinforcementMoment strength at each section must be at least:Moment strength at each section must be at least:Moment strength at each section must be at least:

( )≥ ⋅n n faceM M max .0.25

nM −nM −

n nM M0.5+ −≥



Longitudinal reinforcementLongitudinal reinforcement

Lap splices are permitted if hoops are provided throughout the splice length. Maximum hoop spacing must not exceed d/4 or 100 mm.

No lap splices are permitted in joints or within 2hof column face or where inelastic action is

t d

Lap splices are permitted if hoops are provided throughout the splice length. Maximum hoop spacing must not exceed d/4 or 100 mm.

No lap splices are permitted in joints or within 2hof column face or where inelastic action is

t dexpected.expected.


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Hoops must be provided:Hoops must be provided:

50 mm50 mm 50 mm50 mms ≤d/2s ≤d/2

2h2h 2h2hconfinement

zonesconfinement

zones



Shear design:Shear design:ΔΔ

nn

ΔVeΔVe

prM −prM +

( ) ( )M M+ ( ) ( )M M +( ) ( )pr prizq dere

n

M MV . .

+ −+Δ =

( ) ( )pr prizq dere

n

M MV . .

− ++Δ =

Mpr computed using fypr = 1.25 fy and φ = 1.0Mpr computed using fypr = 1.25 fy and φ = 1.0



Pu1Pu1 Pu2Pu2WuWu

(Vu)ver. right(Vu)ver. right(Vu)vert. left(Vu)vert. left

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xx

Vu(x)Vu(x)

( ) ( )u u uver.izq. ver.der. n

1V V P⎡ ⎤+ − ∑⎢ ⎥⎣ ⎦(Vu)ver. left + ΔVe(Vu)ver. left + ΔVe

(Vu)ver. left- ΔVe(Vu)ver. left- ΔVe

(V ) ΔV(V ) ΔVshear

envelopeshear

envelope (Vu)ver. right+ ΔVe(Vu)ver. right+ ΔVe

(Vu)vert. right - ΔVe(Vu)vert. right - ΔVe

For design, Vc = 0 if ΔVe is more than 50% of required shear strength, or axial force is less than 0.05f’cAg

For design, Vc = 0 if ΔVe is more than 50% of required shear strength, or axial force is less than 0.05f’cAg

21.6 — Special moment frame members subjected to bending and axial load


General

Axial force greater thanThe least section dimension that passes th h th t id t b t th

General

Axial force greater thanThe least section dimension that passes th h th t id t b t th

0.10 ′⋅ ⋅c gf A

through the centroid must be greater than 300 mm.Ratio b/h > 0.4

through the centroid must be greater than 300 mm.Ratio b/h > 0.4


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Column flexural strength must comply with:Column flexural strength must comply with:

1.2≥∑ ∑nc nbM M

Mnc M Mnc M

Mnb

Mnb

Mnc

Mnb

MnbMnc

ncMnb

Mnc

Mnc Mnb

Mnc

Mnc MnbMncMnbMnc

(a) (c)(b)



Transverse reinforcement in confining zones must comply Transverse reinforcement in confining zones must comply g p ywith:Spiral columns:

Columns with hoops:

g p ywith:Spiral columns:

Columns with hoops:

0.3 1⎡ ⎤′ ⎛ ⎞⋅ ⋅ ⋅⎢ ⎥⎜ ⎟

gc c As b fA

0.12′

= ⋅ cs

yt

ff

ρ

1= ⋅ −⎢ ⎥⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦

gc csh

yt chA

f A

0.09 ′⋅ ⋅ ⋅= c c

shyt

s b fAf



hx hx hx

xh mm350≤joint transverse i f t

joint transverse i f tx x x

hx b

hc

b

bs d long.

s0

/ 46

⎧⎪≤ ⎨⎪⎩

confinement zones

confinement zones

50 mm50 mm0

lap splices in central zone

lap splices in central zone

reinforcement as required by 21.7reinforcement as required by 21.7

n

hh 60

⎧⎪≤ ⎨⎪350 h⎧ ⎛ ⎞

⎧≤ ⎨

b long6d .s

50 mm50 mm0

joint transverse reinforcement as required by 21.7

joint transverse reinforcement as required by 21.7

450 mm⎪⎩x350-h100

smmmm

03

150100

⎧ ⎛ ⎞+⎪ ⎜ ⎟⎪ ⎝ ⎠= ⎨≤⎪⎪≥⎩

≤ ⎨⎩

s150 mm



Shear designShear design MprMpr

( ) ( )

Mpr corresponds to the maximum moment strength for the axial load range on theelement (1.25fy and φ=1). Ve cannot be less than the one obtained from analysis.

Mpr corresponds to the maximum moment strength for the axial load range on theelement (1.25fy and φ=1). Ve cannot be less than the one obtained from analysis.

hnhnVeVe

( ) ( )pr prarriba abajoe

n

M MV

h

+=

less than the one obtained from analysis.

For design Vc = 0 if Ve is more than 50% of the required shear or the axial force is less than 0.05f’cAg

less than the one obtained from analysis.

For design Vc = 0 if Ve is more than 50% of the required shear or the axial force is less than 0.05f’cAg

MprMpr


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21.7 — Joints of special moment frames21.7 — Joints of special moment frames

General requirementsGeneral requirements

When computing shear strength within the joint in special frames all longitudinal reinforcement must be presumed to be stressed at 1.25fy.Longitudinal reinforcement terminating at a joint must be extended to the far face of the column confined core and anchored in tension.Wh th b l it di l i f t

When computing shear strength within the joint in special frames all longitudinal reinforcement must be presumed to be stressed at 1.25fy.Longitudinal reinforcement terminating at a joint must be extended to the far face of the column confined core and anchored in tension.Wh th b l it di l i f tWhen the beam longitudinal reinforcement passes through the joint , the column dimension parallel to the reinforcement cannot be less than 20db largest longitudinal bar, for normal weight concrete and 26dbfor lightweight concrete.

When the beam longitudinal reinforcement passes through the joint , the column dimension parallel to the reinforcement cannot be less than 20db largest longitudinal bar, for normal weight concrete and 26dbfor lightweight concrete.

21.7 — Joints of special moment frames21.7 — Joints of special moment framesComputation of the shear demand on the joint:Computation of the shear demand on the joint:

Mpr-cMpr-cVe-colVe-colplane to evaluate

shear Vu

plane to evaluate shear Vu

columncolumn

MM

uu

beambeam

s y sT f A1.25=

c s y sC T f A1.25= =

c s y sC T f A1.25′ ′ ′= =

s y sT f A1.25′ ′=

Ve-colVe-colMpr-cMpr-c

( ) ( )u y s s eviga colV f A A V1.25 ′= + −( ) ( )

( ) ( )

y s eviga col

u

y s eviga col

f A V

Vf A V

1.25

1.25

⎧ −⎪⎪≥ ⎨⎪ ′ −⎪⎩

Beam in both sides:Beam in both sides: Beam in one side:Beam in one side:


Shear strengthShear strength

Joints confined in all four faces

Joints confined in three faces or in opposite faces

Joints confined in all four faces

Joints confined in three faces or in opposite faces

n c jV f A1.70φ φ ′⋅ = ⋅ ⋅ ⋅

n c jV f A1.25φ φ ′⋅ = ⋅ ⋅ ⋅

Other joints Other joints

n c jV f A1.00φ φ ′⋅ = ⋅ ⋅ ⋅

21.7 — Joints of special moment frames21.7 — Joints of special moment framesDefinition of AjDefinition of Aj

Ajbwbw

bwbw

hh

ww

Aj

bwbw

hh

x

w

w

b xb h

2+⎧≤ ⎨ +⎩


13


Development for hooks embedded in the Development for hooks embedded in the confined coreconfined core

db

dh

critical section

y bdh

f d⋅=dh

cf5.4 ′

21.8 — Special moment framesconstructed using precast concrete


The requirements of 21.8 apply for special The requirements of 21.8 apply for special q pp y pmoment frames built using precast concrete forming part of the seismic-force-resistant system.The detailing provisions in 21.8.2 and 21.8.3 are intended to produce frames that respond to design displacements essentially like monolithic special moment

q pp y pmoment frames built using precast concrete forming part of the seismic-force-resistant system.The detailing provisions in 21.8.2 and 21.8.3 are intended to produce frames that respond to design displacements essentially like monolithic special moment esse t a y e o o t c spec a o e tframes.The provisions of 21.8.4 indicate that when not satisfying 21.8.2 or 21.8.3 they must satisfy the requirements of ACI 374.1

esse t a y e o o t c spec a o e tframes.The provisions of 21.8.4 indicate that when not satisfying 21.8.2 or 21.8.3 they must satisfy the requirements of ACI 374.1



Special precast moment frames with ductile ti t l ith ll

Special precast moment frames with ductile ti t l ith llconnections must comply with all

requirements for special cast-in-place frames and Vn should not be less than 2Ve.

Special precast moment frames with strong connections are intended to experience flexural yielding outside the connections

connections must comply with all requirements for special cast-in-place frames and Vn should not be less than 2Ve.

Special precast moment frames with strong connections are intended to experience flexural yielding outside the connectionsflexural yielding outside the connections.

These requirements are applicable independently of any of these two situations.

flexural yielding outside the connections.

These requirements are applicable independently of any of these two situations.

21.9 — Special structural walls and coupling beams


TerminologyTerminology

hh

VuVuww

hwhw


14

21.9 — Special structural walls and coupling beams – General requirements

21.9 — Special structural walls and coupling beams – General requirementsCoverCover 20 mm

Maximum reinforcement spacingMaximum reinforcement spacing

h

ss

ss

s ≤ 3hs ≤ 450 mm

ss



Flexure designFlexure design

Design for flexure and flexure and axial load for structural walls must be performed using the requirements of Chapter 10.

Concrete and developed longitudinal reinforcement within effective flange widths boundary elements

Design for flexure and flexure and axial load for structural walls must be performed using the requirements of Chapter 10.

Concrete and developed longitudinal reinforcement within effective flange widths boundary elementswithin effective flange widths, boundary elements, and the wall web shall be considered effective.

The effects of openings shall be considered.

within effective flange widths, boundary elements, and the wall web shall be considered effective.

The effects of openings shall be considered.

21.9 - Special structural walls andcoupling beams


Flexure designFlexure designFlexure design

Unless a more detailed analysis is performed, effective flange widths of flanged sections ( I, L, Cor T) may be supposed to extend from the face of the web a distance equal to the smaller of:

Flexure design

Unless a more detailed analysis is performed, effective flange widths of flanged sections ( I, L, Cor T) may be supposed to extend from the face of the web a distance equal to the smaller of:

(a) 1/2 the distance to an adjacent wall web, and

(b) 25 percent of the total wall height.

(a) 1/2 the distance to an adjacent wall web, and

(b) 25 percent of the total wall height.



21 9 2 Reinforcement21 9 2 Reinforcement21.9.2 – ReinforcementThe distributed web reinforcement ratios, ρtand ρ , for structural walls shall not be less than 0.0025, except that if Vu does not exceed

(MPa) = (kgf/cm2), ρt and ρ , may be reduced to the values given

21.9.2 – ReinforcementThe distributed web reinforcement ratios, ρtand ρ , for structural walls shall not be less than 0.0025, except that if Vu does not exceed

(MPa) = (kgf/cm2), ρt and ρ , may be reduced to the values given

cv c0.083A f ′λ cv c0.27A f ′λ

in14.3.Separation of reinforcement must not exceed 450 mm

in14.3.Separation of reinforcement must not exceed 450 mm


15

Minimum steel ratioMinimum steel ratio14.3.2 – Minimum steel ratio of vertical reinforcement ρcomputed over gross section is:

0 0012 f f º ( / ) ó

14.3.2 – Minimum steel ratio of vertical reinforcement ρcomputed over gross section is:

0 0012 f f º ( / ) ó0.0012 for deformed bars not larger than Nº 5 (5/8”) ó 16M(16 mm), with fy not less than 420 MPa.

0.0015 for other deformed bars.

0.0012 for welded wire reinforcement with diameter notlarger than16 mm.

14.3.3 - Minimum ratio of horizontal reinforcement area togross concrete area, ρt:

0.0012 for deformed bars not larger than Nº 5 (5/8”) ó 16M(16 mm), with fy not less than 420 MPa.



14.3.3 - Minimum ratio of horizontal reinforcement area togross concrete area, ρt:







Difference between wall and column

Difference between wall and column

14.3.6 — Vertical reinforcement need not be enclosed by lateral ties if vertical reinforcement area is not greater than 0.01 times gross concrete area, or where vertical

14.3.6 — Vertical reinforcement need not be enclosed by lateral ties if vertical reinforcement area is not greater than 0.01 times gross concrete area, or where vertical reinforcement is not required as compression reinforcement.reinforcement is not required as compression reinforcement.



At l t t t i f i f tAt l t t t i f i f tAt least two curtains of reinforcement

must be used in a wall if Vu exceeds

(MPa) =

At least two curtains of reinforcement

must be used in a wall if Vu exceeds

(MPa) = cv c0.17 A f ′λ cv c0.53 A f ′λ

(kgf/cm2)(kgf/cm2)



Nominal shear strength must not exceed:Nominal shear strength must not exceed:o a s ea st e gt ust ot e ceed

where αc is:

o a s ea st e gt ust ot e ceed

where αc is:

n cv c c n yV A f fα λ ρ⎡ ⎤′= +⎣ ⎦

αcαc

0.250.25

w

w

h

0.170.17

00 2.02.01.51.50.50.5 1.01.0 2.52.5


16

Wall boundary elementsWall boundary elements

Boundary elements must be placed at edges and around openings when inelastic response is expected ACI 318-08 gives two alternatives to

Boundary elements must be placed at edges and around openings when inelastic response is expected ACI 318-08 gives two alternatives toexpected. ACI 318-08 gives two alternatives to define if boundary elements are needed: 1) Section 21.9.6.2 presents a displacement-

based procedure. Boundary elements are needed or not depending on the compressive strain at the edge of wall caused by the seismic lateral deflection, or

expected. ACI 318-08 gives two alternatives to define if boundary elements are needed: 1) Section 21.9.6.2 presents a displacement-

based procedure. Boundary elements are needed or not depending on the compressive strain at the edge of wall caused by the seismic lateral deflection, or

2) Section 21.9.6.3 requires boundary elements when the compressive stress at the edge of wall caused by the seismic forces exceeds a threshold value.

2) Section 21.9.6.3 requires boundary elements when the compressive stress at the edge of wall caused by the seismic forces exceeds a threshold value.

Displacement-based boundary element procedure in ACI 318 (21.9.6.2)


This procedure is based on the compressive strain demand at edges of wall when the wall isThis procedure is based on the compressive strain demand at edges of wall when the wall isstrain demand at edges of wall when the wall is deformed under the maximum expected lateral displacement caused by the design earthquake ground motion. Section 21.9.6.2 is based on the assumption that inelastic response of the wall is dominated by flexural action at a critical, yielding section.

strain demand at edges of wall when the wall is deformed under the maximum expected lateral displacement caused by the design earthquake ground motion. Section 21.9.6.2 is based on the assumption that inelastic response of the wall is dominated by flexural action at a critical, yielding section. The wall should be proportioned so that the critical section occurs at the base of the wall and is applicable only to walls continuous from base to top of the structure.

The wall should be proportioned so that the critical section occurs at the base of the wall and is applicable only to walls continuous from base to top of the structure.



The wall should have a single critical section under flexure and axial load at the The wall should have a single critical section under flexure and axial load at the base of the wall.The zones of the wall in compression must be provided with specially reinforced boundary elements when the depth of the neutral axis at nominal strength, c, is greater than:

base of the wall.The zones of the wall in compression must be provided with specially reinforced boundary elements when the depth of the neutral axis at nominal strength, c, is greater than:

and and w

u

w

c600

h

≥⎛ ⎞δ

⋅ ⎜ ⎟⎝ ⎠

u

w0.007

hδ

≥

Nonlinear response of a wallNonlinear response of a wallδδPP

θθ pp

Wall sectionWall section

00 00pp

MuMu MyMy McrMcr φφ uu φφcrcrφφ yyMomentMoment CurvatureCurvature

Plastification lengthPlastification length


17

Nonlinear response of wallNonlinear response of wallUsing Moment-area theorems it is possible to show that the lateral deflection caused by curvature up to yield (green zone) is:

Using Moment-area theorems it is possible to show that the lateral deflection caused by curvature up to yield (green zone) is: bb

and the additional deflection caused by nonlinear rotation (orange zone) is:and the additional deflection caused by nonlinear rotation (orange zone) is:

Total lateral deflection is then:Total lateral deflection is then:

φyφy(φu− φy)(φu− φy)

pp

φuφu

φφ aa

Nonlinear wall deflectionNonlinear wall deflectionww

Curvature at yield

Deflection at yield

Nonlinear deflection

Nonlinear curvature

δyδy (δu−δy)(δu−δy)

hwhw

φφ (φ φ )(φ φ )

pp θpθp

The total deflection is:

We can solve for the ultimate curvature demand and obtain:

The total deflection is:

We can solve for the ultimate curvature demand and obtain:

φyφy (φu − φy)(φu − φy)

Moment-curvature diagram for wall sectionMoment-curvature diagram for wall sectionMM

MnMn

Ultimate curvature demandUltimate curvature demand

φφ

McrMcr

00φcrφcr φyφy φuφuφnφn

What happens at section?What happens at section?

εcuεcuAt level of displacement At level of displacement φφ

ccεs > εyεs > εy

At level ofAt level of

At level of nominal strength

At level of nominal strength

demanddemandStrainStrain

εc = 0.003εc = 0.003

ccεs = εyεs = εy

εc < 0.003εc < 0.003

φuφu

φnφn

φyφy

ww

hh

At level of yield in tension of extreme reinforcement

At level of yield in tension of extreme reinforcement

cycy


18

Equation (21-8) deductionEquation (21-8) deductionThe rotation at the plastic hinge when the displacement demand (δu) takes place is:The rotation at the plastic hinge when the displacement demand (δu) takes place is:

With a plastic hinge length equal to half the wall horizontal length:With a plastic hinge length equal to half the wall horizontal length:

Then the curvature at the wall base when the displacement demand occurs is:Then the curvature at the wall base when the displacement demand occurs is:

Equation (21-8) deductionEquation (21-8) deductionThe concrete strain at the extreme fiber in compression at ultimate is:The concrete strain at the extreme fiber in compression at ultimate is:

We can then obtain the strain at ultimate for the displacement demand:

and

We can then obtain the strain at ultimate for the displacement demand:

and

The value of c for a ultimate strain of εcu = 0.003 is:The value of c for a ultimate strain of εcu = 0.003 is:

Equation (21-8) deductionEquation (21-8) deductionIf a 600 value parameter is used instead of 666 in last equation and it is solved for εcu a value of εcu = 0.0033 is obtained, which in turn leads to the following equation:

If a 600 value parameter is used instead of 666 in last equation and it is solved for εcu a value of εcu = 0.0033 is obtained, which in turn leads to the following equation:

If the maximum strain at the extreme compression fiber exceeds εcu = 0.0033 then the value of c obtained from last equation would be exceeded Thus the form ACI 318 presents it:

If the maximum strain at the extreme compression fiber exceeds εcu = 0.0033 then the value of c obtained from last equation would be exceeded Thus the form ACI 318 presents it:would be exceeded. Thus the form ACI 318 presents it:

If c is greater than the value obtained boundary elements must be placed along the length where it is exceeded and a little more.

would be exceeded. Thus the form ACI 318 presents it:

If c is greater than the value obtained boundary elements must be placed along the length where it is exceeded and a little more.

Need for boundary elements in displacement-based procedureNeed for boundary elements in displacement-based procedure

If equation (21-8) indicates that the value of c is exceeded this is a symptom thatIf equation (21-8) indicates that the value of c is exceeded this is a symptom thatof c is exceeded, this is a symptom that strains greater than εcu = 0.0033 must be expected and the need to confine the edge of the wall is warranted in order to prevent spalling of the concrete there.

of c is exceeded, this is a symptom that strains greater than εcu = 0.0033 must be expected and the need to confine the edge of the wall is warranted in order to prevent spalling of the concrete there.

In that case ACI 318 prescribes the same type and amount of confining transverse reinforcement that for columns.

In that case ACI 318 prescribes the same type and amount of confining transverse reinforcement that for columns.


19

Boundary elements displacement-base procedure

Boundary elements displacement-base procedure

MM

εsεs

εcuεcu

cc

0.0030.003

Region whereRegion where

MnMn

Region where boundary

elements must be provided

Region where boundary

elements must be provided

Displacement-based boundary element procedure in ACI 318 (21.9.6.32

Displacement-based boundary element procedure in ACI 318 (21.9.6.32

Boundary elements must be placed from the critical section up for aBoundary elements must be placed from the critical section up for afrom the critical section up for a distance not less than the larger of w o Mu/(4Vu).The evaluation is performed for the wall when subjected to the nonlinear horizontal design displacements

from the critical section up for a distance not less than the larger of w o Mu/(4Vu).The evaluation is performed for the wall when subjected to the nonlinear horizontal design displacements corresponding to the design earthquake. The value of δu corresponds to the nonlinear roof horizontal displacement.

corresponding to the design earthquake. The value of δu corresponds to the nonlinear roof horizontal displacement.

Stress-based boundary element procedure in ACI 318 (21.9.6.3)

Stress-based boundary element procedure in ACI 318 (21.9.6.3)

Boundary elements must be provided at edges and around openings of walls when the maximum Boundary elements must be provided at edges and around openings of walls when the maximum

c0.2 f ′

p gstress at the extreme fiber in compression caused by factored loads that include seismic effects exceeds unless that whole wall is confined as a column.

p gstress at the extreme fiber in compression caused by factored loads that include seismic effects exceeds unless that whole wall is confined as a column.

u u wcu c

g w

P Mf 0.2 f

A I 2⋅

′= + > ⋅⋅

The boundary elements can be discontinued when the compression stress is less than The boundary elements can be discontinued when the compression stress is less than c0.15 f ′

g w

Pu

Stress-based boundary element procedure in ACI 318 (21 9 6 3)

Stress-based boundary element procedure in ACI 318 (21 9 6 3)

( )u u

cuw

P MP

2 300 mm= +

−

Mu

( )u u

tug w

P MP 0

A 300 mm= − ≤

−

ACI 318 (21.9.6.3)ACI 318 (21.9.6.3)

This procedure had been part of ACI 318 since the 1971 versionThis procedure had been part of ACI 318 since the 1971 versionthe 1971 version. In the 1999 version of 318 a modification was introduced in which the need to resist all flexural forces from seismic effects with just the boundary elements was suppressed.

the 1971 version. In the 1999 version of 318 a modification was introduced in which the need to resist all flexural forces from seismic effects with just the boundary elements was suppressed.


20

Old (pre-1999) procedureOld (pre-1999) procedure

Boundary elements i ti ll fl l

Boundary elements i ti ll fl l

w

resisting all flexural effect that include seismic forces

resisting all flexural effect that include seismic forces

P M

Pu

Mu

heb

( )u u

cuw eb

P MP

2 h= +

−( )u u

tug w eb

P MP 0

A h= − ≤

−

0n c g st st yP [0.85 f (A A ) A f ]′φ ⋅ = φ ⋅ ⋅ ⋅ − + ⋅

n(max) 0nP 0.80 Pφ ⋅ ≤ ⋅ φ ⋅tn st yP A fφ ⋅ = φ ⋅ ⋅



Boundary elements – Both proceduresWhen boundary elements are needed (under any of the two

Boundary elements – Both proceduresWhen boundary elements are needed (under any of the twoWhen boundary elements are needed (under any of the two procedures) these boundary elements must extend horizontally from the maximum compression fiber a distance equal to the greater of : c-0.1 w or c/2.In section with flanges the boundary element must include the effective flange width and must extend at least 300 mminto the web.Transverse reinforcement must be that required for column, but there is no need to comply with equation 21-3.

When boundary elements are needed (under any of the two procedures) these boundary elements must extend horizontally from the maximum compression fiber a distance equal to the greater of : c-0.1 w or c/2.In section with flanges the boundary element must include the effective flange width and must extend at least 300 mminto the web.Transverse reinforcement must be that required for column, but there is no need to comply with equation 21-3.p y qSpecial transverse reinforcement in the boundary element must extend into the foundation element supporting the wall.Wall horizontal transverse reinforcement must be anchored into the confined boundary element core.

p y qSpecial transverse reinforcement in the boundary element must extend into the foundation element supporting the wall.Wall horizontal transverse reinforcement must be anchored into the confined boundary element core.



In ACI 318-08, there are modifications in the requirements for coupling beams inIn ACI 318-08, there are modifications in the requirements for coupling beams inthe requirements for coupling beams in walls.the requirements for coupling beams in walls.

Coupling beamsCoupling beams


21

21.10 — Special structural wallsconstructed using precast concrete

21.10 — Special structural wallsconstructed using precast concrete

Scope— These requirements apply to special t t l ll t t d i t

Scope— These requirements apply to special t t l ll t t d i tstructural walls constructed using precast

concrete forming part of the seismic-force-resisting system.

Special structural walls constructed using precast concrete shall satisfy all requirements of special cast-in-place structural walls plus those of section 21.10.

structural walls constructed using precast concrete forming part of the seismic-force-resisting system.

Special structural walls constructed using precast concrete shall satisfy all requirements of special cast-in-place structural walls plus those of section 21.10.

Special structural walls constructed using precast concrete and unbonded post-tensioning tendons and not satisfying the requirements of 21.10.2 are permitted provided they satisfy the requirements of ACI ITG-5.1.

Special structural walls constructed using precast concrete and unbonded post-tensioning tendons and not satisfying the requirements of 21.10.2 are permitted provided they satisfy the requirements of ACI ITG-5.1.

21.11 — Structural diaphragms and trusses21.11 — Structural diaphragms and trusses

floor diaphragmfloor diaphragm

prescribed horizontal forces

prescribed horizontal forces

21.11 — Structural diaphragms and trusses21.11 — Structural diaphragms and trusses

This section contains:This section contains:

Requirements for slabs-on-grade , floor and roof slabs when they are part of the seismic-force-resisting system must comply with this section. Minimum thickness for diaphragms are given.Gives minimum reinforcement for diaphragms.Indicates shear strength for these elementsDefines when boundary elements must be used in

Requirements for slabs-on-grade , floor and roof slabs when they are part of the seismic-force-resisting system must comply with this section. Minimum thickness for diaphragms are given.Gives minimum reinforcement for diaphragms.Indicates shear strength for these elementsDefines when boundary elements must be used inDefines when boundary elements must be used in diaphragms.Includes requirements for construction joints within the diaphragm.

Defines when boundary elements must be used in diaphragms.Includes requirements for construction joints within the diaphragm.


22

21.12 — Foundations21.12 — FoundationsThis section contains:

21.12.1 — Scope - Foundations resisting earthquake i d d f f i h k i d d

This section contains:21.12.1 — Scope - Foundations resisting earthquake i d d f f i h k i d dinduced forces or transferring earthquake-induced forces between structure and ground.21.12.2 — Footings, foundation mats, and pile caps –Gives requirements for the anchoring of reinforcement in vertical elements of the seismic-force-resisting system to these foundation elements. 21.12.3 — Grade beams and slabs-on-ground – Sets minimum dimension s and minimum reinforcement for

induced forces or transferring earthquake-induced forces between structure and ground.21.12.2 — Footings, foundation mats, and pile caps –Gives requirements for the anchoring of reinforcement in vertical elements of the seismic-force-resisting system to these foundation elements. 21.12.3 — Grade beams and slabs-on-ground – Sets minimum dimension s and minimum reinforcement for u d e s o s a d u e o ce e t othese elements,21.12.4 — Piles, piers, and caissons – Indicates the type of effects to take into account in design and the minimum reinforcement allowable for these elements.

u d e s o s a d u e o ce e t othese elements,21.12.4 — Piles, piers, and caissons – Indicates the type of effects to take into account in design and the minimum reinforcement allowable for these elements.

21.13 — Members not designated as part ofthe seismic-force-resisting system


This section is a response to the extended practice by structural designers of designating arbitrarily some of the This section is a response to the extended practice by structural designers of designating arbitrarily some of the structural designers of designating arbitrarily some of the structural elements as being parte of the seismic-force-resisting system and part not. Northridge Earthquake affecting the City of Los Angeles in 1994 pointed out great deficiencies in this practice. In ACI 318-95 this section was totally revised and it was updated in 1999, 2002, 2005 , and now in 2008.

In essence it is a call to the designer to check the

structural designers of designating arbitrarily some of the structural elements as being parte of the seismic-force-resisting system and part not. Northridge Earthquake affecting the City of Los Angeles in 1994 pointed out great deficiencies in this practice. In ACI 318-95 this section was totally revised and it was updated in 1999, 2002, 2005 , and now in 2008.

In essence it is a call to the designer to check theIn essence it is a call to the designer to check the deformation levels that so called “non participating” elements are subjected and the minimum reinforcement they should comply with.

In essence it is a call to the designer to check the deformation levels that so called “non participating” elements are subjected and the minimum reinforcement they should comply with.



This Section contains two procedures to check non-This Section contains two procedures to check non-pparticipating elements that are not part of the seismic-force-resisting system:

• When the forces induced by the design displacement combined with the gravity forces do not exceed the design strength of the elements, this section indicates the minimum reinforcement to use.

pparticipating elements that are not part of the seismic-force-resisting system:

• When the forces induced by the design displacement combined with the gravity forces do not exceed the design strength of the elements, this section indicates the minimum reinforcement to use.

• If the strength is exceeded the sections of Chapter 21 that are mandatory for these elements are indicated.

• If the strength is exceeded the sections of Chapter 21 that are mandatory for these elements are indicated.



This section includes new requirements for slab-This section includes new requirements for slab-This section includes new requirements for slab-column frames that are not part of the seismic-force-resisting system.

Slab-column frames have shown repeatedly their vulnerability under seismic demands. This vulnerability is specially associated with the punching shear strength of the slab-column joint.

This section includes new requirements for slab-column frames that are not part of the seismic-force-resisting system.

Slab-column frames have shown repeatedly their vulnerability under seismic demands. This vulnerability is specially associated with the punching shear strength of the slab-column joint.

The new procedure in ACI 318-08 (Section 21.13.6) indicates when shear reinforcement must be provided in the slab-column joint as a function of the story drift.

The new procedure in ACI 318-08 (Section 21.13.6) indicates when shear reinforcement must be provided in the slab-column joint as a function of the story drift.


23



Story drift cannot exceed the larger of: Story drift cannot exceed the larger of:

⎛ ⎞−⎜ ⎟

⎝ ⎠

ug

c

orVV

0.005

0.035 0.05φ

where Vug is the factored gravity punching shear demand and Vc is the punching shear strength.

where Vug is the factored gravity punching shear demand and Vc is the punching shear strength.

⎝ ⎠



The EndThe End

ACI 318 08 Seismic Requirements L E Garcia

Documents

intermediate moment framestwo

energy dissipation capacity

special moment frames

seismic design category

special structural walls

punching shear strength

resisting systemthis section

joint transverse reinforcement